The present invention relates to the provision of a highly stable master clock to supply the timing in a time division multiple access (TDMA) system. In the following the application of the invention to baseband processing satellite switched TDMA systems will be described, but this does not exclude its application to TDMA systems in general.
Time division multiple access (TDMA) is a multiple access technique that permits individual earth terminal transmissions to be received by a satellite in separate non-overlapping time slots. Each TDMA earth station has parallel input digital bitstreams, or analog streams that are digitized at the earth station, which are addressed to separate receiving earth stations. The signals being addressed to these stations are allocated separate portions of the transmit TDMA burst following the TDMA burst preamble.
Most communication satellites simply relay the received TDMA signal, the only signal processing operations being filtering, amplification and frequency shift. With the advent however of satellites equipped with spot beam antennas, it has become necessary to install onboard switching matrices capable of transferring uplink bursts, addressed to a specific ground station, to the appropriate spot beam antenna. These systems are referred to as satellite switched TDMA systems or, briefly, SS-TDMA. The satellite switching operations involved are either carried out at intermediate frequency or at baseband. Baseband switching has the advantage that the TDMA signal can be stored and thus the TDMA frame can be reorganized prior to transmission.
One of the major problems in interconnecting SS-TDMA systems with ground networks consists in the provision of a highly stable reference clock. In the future, national ground networks will be synchronous. This means that an SS-TDMA satellite payload for national interconnections must track the national clock to allow synchronous interconnections. Furthermore, interconnections between different national synchronous networks require an onboard master clock having a long term stability of 10.sup.-11, giving one PCM frame slip (125 microsec) per 72 day, as stipulated by recommendation G 811 "Plesiochronous Operation of International Digital Links" (CCITT Orange Book, Vol. III-2, 1977).